contrib/credential: fix compilation of "osxkeychain" helper

The "osxkeychain" helper does not compile due to a warning generated by
the unused `argc` parameter. Fix the warning by checking for the minimum
number of required arguments explicitly in the least restrictive way
possible.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

contrib/credential/osxkeychain/git-credential-osxkeychain.c

@@ -422,7 +422,7 @@ int main(int argc, const char **argv)
 	const char *usage =
 		"usage: git credential-osxkeychain <get|store|erase>";
 
-	if (!argv[1])
+	if (argc < 2 || !*argv[1])
 		die("%s", usage);
 
 	if (open(argv[0], O_RDONLY | O_EXLOCK) == -1)